Re: [CAD_CAM_EDM_DRO] RE: Relinear encoder source: Now Limbo Rock

Ray wrote:

> From: "Carol & Jerry Jankura" <jerry.jankura@...>
>
> Peter:
>
> >I've worked with some of the CompuMotor servo systems. The equivalent of
> >"missing steps" with a servo motor is having the difference between actual
> >shaft angle and the "computed" shaft angle be larger than some allowable
> >error. When this occurred, the motor was shut down and allowed to "free
> >wheel" until the controller was reset. One line from the controller back to
> >the computer was set to logic zero to alert the computer of the problem.
> >Your software had to detect this line and take appropriate action. With
> >CompuMotor controllers, this was a "wired-or" connection so several motor
> >controller failures could be checked by monitoring one line.
>
> Diference between commanded and actual position is called following error.
> In servo systems, this number is normally computed at the point where the
> control loop between motor amp commands and feedback system (resolver,
> encoder, scale) is closed. This will be a different place depending on
> what system you are using.
>
> In the servo gecko drive amps, this is in the drive itself. If the encoder
> gets more than a few steps behind the step signals from the computer, the
> gecko trips out. I think that this condition is indicated on screw five.
> There are a number of commercial servo amps that are driven by digital
> signals that will behave roughly the same.
>
> In the servo EMC system and many proprietary controls the loop is closed
> in the PC or control computer itself. The motor drive amp is just that,
> it amplifies the command signal and sends it to the motor. There will be
> a fault line for overvoltage and overcurrent, but this kind of amp does not
> signal any difference between commanded and actual position.
>
> When the motion loop is closed in the computer, as it is in the EMC,
> following error is settable in the ini file or by machine parameters and can
> be a ramp proportional to speed. It can also be different for each axis.
> In this case the PC signals a following error problem to the rest of the
> system and changes its mode to machine off or e-stop.
>
> One major difference between stepper and servo systems with respect to
> following error is that in a servo system, the greater the following error
> the greater will be the command signal to the motor amp in an attempt to
> reduce following error.

There is one other feature of a true servo system, where the encoder feeds back into
the computer. The computer can also be used as a DRO in manual operation, and
the computer does not lose position information during an emergency stop
or following error trip.

With step/dir to servo drives, as well as all open-loop stepper systems, any
axis that is moving when E-stop is pushed will be at some unknown position,
and if any axis stalls, its position will be unknown.

Jon